Nonaqueous electrolyte secondary batteries

ABSTRACT

A nonaqueous electrolytic solution for nonaqueous electrolyte secondary batteries includes a nonaqueous solvent and an electrolyte. The nonaqueous solvent includes a fluorinated carboxylate ester represented by the formula (1): 
     
       
         
         
             
             
         
       
     
     where R 1  and R 2  are each any of H, F, CH 3-x F x  (x is 1, 2 or 3) and R 3  is an optionally fluorinated alkyl group having 1 to 3 carbon atoms. The nonaqueous electrolytic solution further comprising lithium fluorosulfate salt (LiSO 3 F).

BACKGROUND

1. Technical Field

The present disclosure relates to nonaqueous electrolytic solutions fornonaqueous electrolyte secondary batteries and to nonaqueous electrolytesecondary batteries.

2. Description of the Related Art

A nonaqueous electrolytic solution used in nonaqueous electrolytesecondary batteries includes a nonaqueous solvent and an electrolytesalt. Non-Patent Literature 1 (ECS Transactions, 11 (29) 91-98 (2008),Electrolytes Containing Fluorinated Ester Co-Solvents forLow-Temperature Li-Ion Cells) discloses that the use of a nonaqueoussolvent which includes a fluorinated carboxylate ester provides goodlow-temperature discharge characteristics. Further, Patent Literature 1(Japanese Patent No. 5235437) discloses that high-temperature storagecharacteristics are improved by using a nonaqueous solvent whichincludes a fluorinated carboxylate ester having a hydrogen atom at theα-position.

In the techniques disclosed in Non-Patent Literature 1 and PatentLiterature 1, the fluorinated carboxylate ester is decomposed by analkali component present in the positive electrode during the firstcharging and the decomposition product is diffused toward the negativeelectrode to form an excessively thick film. The film inhibits thedeintercalation of lithium ions from the negative electrode duringdischarging to cause a decrease in initial efficiency. In particular,the decrease in initial efficiency is marked in the case of a Ni-richpositive electrode active material because the active material containsa large amount of an alkali component.

SUMMARY

In one general aspect, the techniques disclosed here feature anonaqueous electrolytic solution for nonaqueous electrolyte secondarybatteries including a nonaqueous solvent and an electrolyte, thenonaqueous solvent including a fluorinated carboxylate ester, thenonaqueous electrolytic solution containing lithium fluorosulfate salt(LiSO₃F).

The nonaqueous electrolytic solutions for nonaqueous electrolytesecondary batteries includes a nonaqueous solvent and an electrolyte.The nonaqueous solvent includes a fluorinated carboxylate esterrepresented by the formula (1):

where R₁ and R₂ are each any of H, F, CH_(3-x)F_(x) (x is 1, 2 or 3) andR₃ is an optionally fluorinated alkyl group having 1 to 3 carbon atoms.The nonaqueous electrolytic solution further comprising lithiumfluorosulfate salt (LiSO₃F).

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating the appearance of a nonaqueouselectrolyte secondary battery representing an embodiment of the presentdisclosure;

FIG. 2 is a sectional view taken along line II-II in FIG. 1;

FIG. 3 is a view illustrating the outer bottom of the nonaqueouselectrolyte secondary battery in the above embodiment; and

FIG. 4 is a view illustrating the inner bottom of the nonaqueouselectrolyte secondary battery in the above embodiment.

DETAILED DESCRIPTION

In a nonaqueous electrolyte secondary battery, the addition of afluorinated carboxylate ester to a nonaqueous electrolytic solutionincluding a nonaqueous solvent and an electrolyte salt leads to goodlow-temperature discharge characteristics. Further, the battery achievesan improvement in high-temperature storage characteristics in a chargedstate when the nonaqueous electrolytic solution contains a fluorinatedcarboxylate ester having a hydrogen atom at the α-position. It istherefore desirable that the nonaqueous electrolytic solution contain afluorinated carboxylate ester having an α hydrogen atom in order toallow the battery to attain good low-temperature dischargecharacteristics and high-temperature storage characteristics.

It is known that in a nonaqueous electrolyte secondary battery, part ofthe nonaqueous electrolytic solution is decomposed during the firstcharging and the decomposition product forms a film on the surface ofthe negative electrode. The surface of the negative electrode refers tothe interface contributing to the reaction that is formed between thenonaqueous electrolytic solution and the negative electrode activematerial, and specifically indicates the surface of the negativeelectrode active material layer and the surface of the negativeelectrode active material. Such a film is called an SEI (solidelectrolyte interface) film and works in favor of batterycharacteristics.

If the decomposition product from the nonaqueous electrolytic solutionforms an excessively thick film on the surface of the negativeelectrode, lithium ions that have been intercalated into the negativeelectrode during the first charging are inhibited from beingdeintercalated during discharging and consequently the initialefficiency is decreased. The initial efficiency may be represented bythe following equation:

Initial efficiency=First discharge capacity/First charge capacity×100

The above decrease in initial efficiency is encountered when thenonaqueous electrolytic solution contains a fluorinated carboxylateester with an α-hydrogen atom represented by the following generalformula (1). In particular, the decrease is marked when a high-Nipositive electrode active material containing a large amount of analkali component is used. The reason for this is probably because, asshown in the reaction formula (I) below, the fluorinated carboxylateester is decomposed by the alkali component, for example, lithiumcarbonate present in the positive electrode active material to generateH₂O and R₁R₂C═CHCOOR₃, which are then diffused toward the negativeelectrode to form an excessively thick film on the surface of thenegative electrode.

(In the formula, R₁ and R₂ are each any of H, F, CH_(3-x)F_(x) (x is 1,2 or 3) and may be the same as or different from each other, and R₃ isan optionally fluorinated alkyl group having 1 to 3 carbon atoms.)

Li₂CO₃+2R₁R₂FCCH₂COOR₃→2LiF+CO₂+H₂O+2R₁R₂C═CHCOOR₃  Reaction Formula (I)

The present inventors conducted extensive studies in order to solve theproblems discussed above. As a result, the present inventors have foundthat lithium fluorosulfate salt (LiSO₃F) disclosed as being effectivefor enhancing low-temperature characteristics in Japanese UnexaminedPatent Application Publication No. 2011-187440 should be added to anonaqueous electrolytic solution which contains a fluorinatedcarboxylate ester having an α hydrogen atom, and have completed atechnique that forms a basis of an embodiment of the present disclosure.In such an embodiment, LiSO₃F is adsorbed to the positive electrode soas to suppress the decomposition reaction of the fluorinated carboxylateester represented by the reaction formula (I). Thus, the addition ofLiSO₃F results in an improvement in initial efficiency while stillensuring good low-temperature discharge characteristics and goodhigh-temperature storage characteristics.

Some nonaqueous electrolyte secondary batteries are so configured thatan electrode assembly in which positive and negative electrodes arewound or stacked together through a separator, and a nonaqueouselectrolytic solution are accommodated in an Fe-based exterior case andthe negative electrode is electrically connected to the exterior case.The surface of the exterior case of such a battery is plated withnickel. The present inventors have experienced that when the nonaqueouselectrolytic solution of the battery contains a sulfur compound, forexample, LiSO₃F, the exterior case is corroded during over-dischargingat high temperatures. The reasons for this phenomenon are probably asfollows. Products resulting from the decomposition of the sulfurcompound, for example, LiSO₃F, react with nickel as the skin of theexterior case to cause iron to be exposed. In an over-discharging testat high temperatures, the exterior case is subjected to a potential ofabout 3 V versus lithium, and consequently the exposed iron is dissolvedand the case is corroded.

During the studies directed to reaching the embodiment discussed here,the present inventors have found that the corrosion of the exterior caseis suppressed by adding a fluorinated carboxylate ester together withLiSO₃F to the nonaqueous electrolytic solution. The fluorinatedcarboxylate ester added to the nonaqueous electrolytic solution has ahydrogen atom at the α position and is decomposed as illustrated in thereaction formula (II) below to form a film on the inner peripheral walland the inner bottom of the exterior case. The suppression of thecorrosion of the exterior case is probably ascribed to this film servingas a protective layer for the exterior case. Specifically, theprotective layer (the protective film) suppresses the reaction betweenthe decomposition products derived from LiSO₃F and nickel as the skin ofthe exterior case and thereby prevents iron from being exposed. This isprobably the reason why the dissolution of iron is prevented and thecorrosion of the exterior case is controlled even when anover-discharging test is performed at high temperatures.

Hereinbelow, an embodiment of the present disclosure will be describedin detail with reference to the drawings. The drawings used in thedescription of the embodiment are schematic, and the constituentsillustrated in the drawings are sometimes not to scale. Specific scalessuch as sizes should be estimated in consideration of the descriptiongiven below.

FIG. 1 is a perspective view of a nonaqueous electrolyte secondarybattery representing an example of the present embodiment. FIG. 2 is asectional view taken along line II-II in FIG. 1. In the nonaqueouselectrolyte secondary battery, as illustrated in FIG. 2, an electrodeassembly 4 in which a positive electrode 1 and a negative electrode 2are wound together through a separator 3, and a nonaqueous electrolyticsolution (not shown) are accommodated in a bottomed cylindrical exteriorcase 5. Near the opening of the cylinder, the exterior case 5 has acircumferential groove 5 c that is U-shaped in cross section. A sealingmember 6 serves as a lid and is attached so as to tightly close the openend of the exterior case 5 through a gasket 7.

The sealing member 6 includes a sealing plate 8, a valving member 9, aninner cap 10, an evacuation outlet 11 and a filter 12. The sealing plate8 serves as a positive electrode external terminal. The valving member9, the inner cap 10 and the evacuation outlet 11 serve as a safety valvethrough which any gas generated in the battery is discharged to theoutside of the battery.

The valving member 9 and the inner cap 10 have a thin portion 9 a and athin portion 10 a that are to be broken when the pressure inside thebattery reaches a prescribed value. The evacuation outlet 11 is disposedin the sealing plate 8, and any gas that has been generated in thebattery and has broken the valving member 9 and the inner cap 10 isdischarged to the outside of the battery through the evacuation outlet11. The filter 12 has an opening 12 a for the discharging of the gas.

Insulating plates 13 and 14 are disposed on the sealing member 6 side ofthe electrode assembly 4 and on the other side near the bottom 5 a ofthe exterior case 5, respectively. The positive electrode 1 is connectedto the filter 12 through a positive electrode lead 15, and the negativeelectrode 2 to the bottom 5 a of the exterior case 5 through a negativeelectrode lead 16.

FIG. 3 is a view illustrating the outer surface of the bottom 5 a of theexterior case 5. Desirably, as illustrated in FIG. 3, the bottom 5 a ofthe exterior case 5 has a ring-shaped thin portion 5 b that is to bebroken when the pressure inside the battery reaches a prescribed value.The thin portion 5 b is formed so that the bottom 5 a is recessed fromthe outer bottom surface toward the inner surface of the bottom. Thethin portion 5 b is broken upon the prescribed increase in the pressureinside the battery and thereby serves to prevent the breakage of thecylindrical side wall of the exterior case 5. It is desirable that thethin portion 5 b be configured to operate at a pressure higher than thepressure which initiates the operation of the thin portion 9 a disposedin the valving member 9.

FIG. 4 is a view illustrating the inner surface of the bottom 5 a of theexterior case 5. When the thin portion 5 b is disposed in the bottom 5 aof the exterior case 5, as illustrated in FIG. 4, the negative electrodelead 16 is arranged so that its one end is in a region enclosed by thethin portion 5 b. This end of the negative electrode lead 16 has such alength and a width that the end portion does not interfere with the thinportion 5 b, and thus the operation of the thin portion 5 b will not behindered.

To prevent the corrosion of iron to a greater extent, it is desirablethat the inner surface of the exterior case 5 be plated with nickel.From the point of view of cost, the thickness of the nickel plating isdesirably not more than 2 μm, and more desirably not more than 1 μm onthe inner surface of the exterior case 5. The nonaqueous electrolyticsolution according to the present disclosure does not cause thecorrosion of the exterior case even when at least a portion of thenickel plating on the inner surface has a thickness of 1 μm or less.

Next, the elements constituting the nonaqueous electrolyte secondarybattery will be described in detail.

[Positive Electrodes]

The positive electrode 1 is composed of, for example, a positiveelectrode current collector such as a metal foil, and a positiveelectrode active material layer disposed on the positive electrodecurrent collector. For example, the positive electrode current collectoris a foil of a metal that is stable at a range of potentials applied tothe positive electrode 1, or a film having a skin layer of a metal thatis stable at a range of potentials applied to the positive electrode 1.Aluminum (Al) is desirable as the metal that is stable at a range ofpotentials applied to the positive electrode 1. For example, thepositive electrode active material layer includes a positive electrodeactive material and other components such as a conductive agent and abinder, and is obtained by mixing these components in an appropriatesolvent and applying the mixture onto the positive electrode currentcollector followed by drying and rolling.

The positive electrode active material may be a lithium (Li)-containingtransition metal oxide. Some of the transition metal atoms in thetransition metal oxide may be replaced by atoms of a dissimilar element.The transition metal element may be at least one selected from the groupconsisting of scandium (Sc), manganese (Mn), iron (Fe), cobalt (Co),nickel (Ni), copper (Cu) and yttrium (Y). Of these transition metalelements, desired elements are, for example, manganese, cobalt andnickel. The dissimilar element may be at least one selected from thegroup consisting of magnesium (Mg), aluminum (Al), lead (Pb), antimony(Sb) and boron (B). Of these dissimilar elements, desired elements are,for example, magnesium and aluminum.

Specific examples of the positive electrode active materials includelithium-containing transition metal oxides such as LiCoO₂, LiNiO₂,LiMn₂O₄, LiMnO₂, LiNi_(1-y)Co_(y)O₂ (0<y<1), LiNi_(1-y-z)Co_(y)Mn_(z)O₂(0<y+z<1) and LiNi_(1-y-z)Co_(y)Al_(z)O₂ (0<y+z<1). In particular,LiNi_(1-y-z)Co_(y)Mn_(z)O₂ (0<y+z<0.5) and LiNi_(1-y-z)Co_(y)Al_(z)O₂(0<y+z<0.5) containing nickel in a proportion of not less than 50 mol %relative to all the transition metals are desirable from the points ofview of cost and specific capacity. These positive electrode activematerials contain a large amount of alkali components and thusaccelerate the decomposition of nonaqueous electrolytic solutions tocause a decrease in durability. However, the nonaqueous electrolyticsolution of the present disclosure is resistant to decomposition evenwhen used in combination with these positive electrode active materials.The positive electrode active materials may be used singly, or two ormore may be used in combination.

The conductive agent serves to increase the electron conductivity of thepositive electrode active material layer. Examples of the conductiveagents include conductive carbon materials, metal powders and organicmaterials. Specific examples include such carbon materials as acetyleneblack, Ketjen black and graphite, such metal powders as aluminum powder,and such organic materials as phenylene derivatives. The conductiveagents may be used singly, or two or more may be used in combination.

The binder serves to ensure a good contact between the positiveelectrode active material and the conductive agent and to increase theadhesion of the components such as the positive electrode activematerial with respect to the surface of the positive electrode currentcollector. Examples of the binders include fluoropolymers and rubberypolymers. Specific examples include such fluoropolymers aspolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) andmodified products thereof, and such rubbery polymers asethylene-propylene-isoprene copolymer and ethylene-propylene-butadienecopolymer. The binder may be used in combination with a thickener suchas carboxymethylcellulose (CMC) or polyethylene oxide (PEO).

[Negative Electrodes]

The negative electrode 2 is composed of, for example, a negativeelectrode current collector such as a metal foil, and a negativeelectrode active material layer disposed on the negative electrodecurrent collector. For example, the negative electrode current collectoris a foil of a metal that is not alloyed with lithium at a range ofpotentials applied to the negative electrode 2, or a film having a skinlayer of a metal that is not alloyed with lithium at a range ofpotentials applied to the negative electrode 2. Copper is suitable asthe metal that is not alloyed with lithium at a range of potentialsapplied to the negative electrode 2 because this metal is easilyprocessed at low cost and has good electron conductivity. For example,the negative electrode active material layer includes a negativeelectrode active material and other components such as a binder, and isobtained by mixing these components in water or an appropriate solventand applying the mixture onto the negative electrode current collectorfollowed by drying and rolling.

The negative electrode active materials are not particularly limited aslong as the materials can store and release lithium ions. Examples ofthe negative electrode active materials include carbon materials,metals, alloys, metal oxides, metal nitrides, and lithium-intercalatedcarbon and silicon. Examples of the carbon materials include naturalgraphite, artificial graphite and pitch-based carbon fibers. Specificexamples of the metals and the alloys include lithium (Li), silicon(Si), tin (Sn), germanium (Ge), indium (In), gallium (Ga), lithiumalloys, silicon alloys and tin alloys. The negative electrode activematerials may be used singly, or two or more may be used in combination.

Similarly to the case of the positive electrode 1, the binder may be afluoropolymer or a rubbery polymer and is desirably a rubbery polymersuch as styrene-butadiene copolymer (SBR) or a modified product thereof.The binder may be used in combination with a thickener such ascarboxymethylcellulose (CMC) sodium.

For example, the negative electrode current collector is a foil of ametal that is not alloyed with lithium at a range of potentials appliedto the negative electrode 2, or a film having a skin layer of a metalthat is not alloyed with lithium at a range of potentials applied to thenegative electrode 2. Copper is suitable as the metal that is notalloyed with lithium at a range of potentials applied to the negativeelectrode 2 because this metal is easily processed at low cost and hasgood electron conductivity.

[Separators]

The separator 3 may be a porous film having ion permeability andinsulating properties and is disposed between the positive electrode 1and the negative electrode 2. Examples of the porous films includemicroporous thin films, woven fabrics and nonwoven fabrics. Suitablematerials for the separators are polyolefins. More specifically, forexample, polyethylene and polypropylene are desirable.

[Nonaqueous Electrolytic Solutions]

The nonaqueous electrolytic solution includes a nonaqueous solvent, anelectrolyte salt dissolved in the nonaqueous solvent, and an additive.The nonaqueous electrolytic solution includes a fluorinated carboxylateester having an α hydrogen atom as the nonaqueous solvent, and lithiumfluorosulfate salt (LiSO₃F) as the additive.

Fluorinated chain carboxylate esters are disclosed to react with anegative electrode to be reductively decomposed at about 1.2 V or lessversus metallic lithium (see Japanese Unexamined Patent ApplicationPublication No. 2009-289414). To prevent this reductive decompositionfrom occurring to an excessive degree on the surface of the negativeelectrode, it is desirable to add a film-forming compound to thenonaqueous solvent which can form a film on the surface of the negativeelectrode.

The film-forming compound is suitably fluoroethylene carbonate (FEC)which can form an appropriate film on the surface of the negativeelectrode and also functions effectively as a nonaqueous solvent.

If FEC is added to the nonaqueous solvent in an excessively smallamount, the compound fails to form a sufficient film on the surface ofthe negative electrode and the fluorinated chain carboxylate ester issometimes decomposed reductively to cause a decrease in high-temperaturestorage characteristics. If, on the other hand, the amount of FEC isexcessively large, the nonaqueous electrolytic solution exhibits so higha viscosity that load characteristics are decreased at times. Thus, itis desirable that the amount of FEC be controlled to the range of 2 vol% to 40 vol %, more desirably 5 vol % to 30 vol % relative to the wholenonaqueous solvent.

As the fluorinated carboxylate ester, methyl 3,3,3-trifluoropropionate(FMP) is desirable because this compound has low viscosity and provideshigh conductivity. The fluorinated carboxylate ester is desirablypresent in 50 vol % or more relative to the whole nonaqueous solvent inthe nonaqueous electrolytic solution. This amount ensures that thefluorinated carboxylate ester forms an appropriate film on the surfaceof the negative electrode while serving as a nonaqueous solvent.

When used in combination with the nonaqueous solvent including thefluorinated carboxylate ester, LiSO₃F prevents the reaction of thefluorinated carboxylate ester with an alkali component present in thepositive electrode 1 and thereby enhances the initial efficiency whilegood low-temperature discharge characteristics and high-temperaturestorage characteristics are ensured. LiSO₃F is desirably added in 0.1 to3 mass % relative to the total mass of the nonaqueous electrolyticsolution. If the amount is below the lower limit of this range, LiSO₃Fwill fail to prevent sufficiently the decomposition reaction of thefluorinated carboxylate ester. If the amount exceeds the upper limit ofthe above range, decomposition reaction will occur markedly to cause anincrease in internal resistance and the generation of gas at times.

As described above, the addition of LiSO₃F to the nonaqueous solventincluding the fluorinated carboxylate ester results in a nonaqueouselectrolytic solution which allows a battery to achieve an enhancementin initial efficiency while still exhibiting good low-temperaturedischarge characteristics and high-temperature storage characteristicsby virtue of the action of the fluorinated carboxylate ester.

Further, the combined use of the fluorinated carboxylate ester andLiSO₃F ensures that the resultant nonaqueous electrolytic solution doesnot cause the corrosion of the exterior case 5 of the battery containingthe nonaqueous electrolytic solution. Specifically, the fluorinatedcarboxylate ester undergoes the decomposition reaction represented bythe reaction formula (II) described hereinabove to form a film whichserves as a protective layer for the inner surface of the exterior case5. This protective layer suppresses the reaction between LiSO₃F-deriveddecomposition products and nickel deposited on the inner surface of theexterior case 5. As a result, iron forming the exterior case 5 is notexposed and the corrosion of the case is prevented.

Desirably, the nonaqueous solvent further includes propylene carbonate(PC). Propylene carbonate is easily decomposed on the negative electrodeduring the first charging and reacts coordinately when the fluorinatedcarboxylate ester undergoes the decomposition reaction. In this manner,propylene carbonate and the fluorinated carboxylate ester form a densecomposite film which attains equal or higher effects compared to thefilm formed by the decomposition reaction of the fluorinated carboxylateester alone.

The nonaqueous solvent may include the fluorinated chain carboxylateester, FEC, PC and further a fluorine-free nonaqueous solvent. Desiredfluorine-free nonaqueous solvents are ethylene carbonate (EC) and ethylmethyl carbonate (EMC). Examples of such solvents further includedimethyl carbonate (DMC), diethyl carbonate (DEC), methyl acetate,methyl propionate and ethyl acetate.

The electrolyte salt may be a lithium salt. Examples of the lithiumsalts include LiPF₆, LiBF₄, LiCF₃SO₃, LiClO₄, LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃ and LiC(C₂F₅SO₂)₃,with LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ being particularly desirable.

The nonaqueous electrolytic solution may include an additional additiveother than LiSO₃F. The additives serve as surface film-forming agentswhich form an ion permeable film on the surface of the positiveelectrode 1 or the negative electrode 2 before the decompositionreaction of the nonaqueous solvent and the electrolyte salt occurs onthe surface of the positive electrode 1 or the negative electrode 2 andthereby prevent the decomposition reaction of the nonaqueouselectrolytic solution on the surface of the positive electrode 1 or thenegative electrode 2.

Examples of such additives include vinylene carbonate (VC), ethylenesulfite (ES), lithium bis(oxalato)borate (LiBOB), cyclohexylbenzene(CHB) and ortho-terphenyl (OTP). The additives may be used singly, ortwo or more may be used in combination. The proportion of the additivesin the nonaqueous electrolytic solution is not limited as long as asufficient film can be formed and is desirably greater than 0 mass % andnot more than 3 mass % relative to the total mass of the nonaqueouselectrolytic solution.

In particular, vinylene carbonate is easily decomposed on the negativeelectrode and reacts coordinately when the fluorinated carboxylate esterundergoes the decomposition reaction. In this manner, vinylene carbonateand the fluorinated carboxylate ester advantageously form a densecomposite film.

The nonaqueous electrolytic solution of the present disclosure may beapplied to a nonaqueous electrolyte secondary battery that is soconfigured that an electrode assembly 4 in which a positive electrode 1and a negative electrode 2 are wound or stacked together through aseparator 3, and the nonaqueous electrolytic solution are accommodatedin an Fe-based exterior case 5 and the negative electrode 2 iselectrically connected to the exterior case 5.

Hereinbelow, the present disclosure will be described in greater detailbased on examples and comparative examples. However, the scope of thepresent disclosure is not limited to such examples. In Examples 1 to 5and Comparative Examples 1 to 8, nonaqueous electrolyte secondarybatteries were fabricated in the manner specifically described below.

Example 1 Fabrication of Positive Electrode

Lithium-containing transition metal oxideLiNi_(0.50)Co_(0.20)Mn_(0.30)O₂ was used as a positive electrode activematerial. The active material, acetylene black and polyvinylidenefluoride were mixed together in a mass ratio of 96:2:2. An appropriateamount of N-methyl-2-pyrrolidone (NMP) was added to the mixture. Apositive electrode mixture slurry was thus prepared. Next, the positiveelectrode mixture slurry was applied to both sides of an aluminum foilas a positive electrode current collector. The wet films were dried androlled with a roller. In this manner, a positive electrode 1 wasfabricated which had the positive electrode active material layers onboth sides of the positive electrode current collector. The packingdensity of the positive electrode 1 was 3.4 g/cm³.

[Fabrication of Negative Electrode]

Artificial graphite, carboxymethylcellulose sodium (CMC-Na) andstyrene-butadiene copolymer (SBR) were mixed together in a mass ratio of98:1:1 in an aqueous solution to give a negative electrode mixtureslurry. Next, the negative electrode mixture slurry was uniformlyapplied to both sides of a copper foil as a negative electrode currentcollector. The wet films were dried and rolled with a roller. In thismanner, a negative electrode 2 was fabricated which had the negativeelectrode mixture layers on both sides of the negative electrode currentcollector. The packing density of the negative electrode 2 was 1.6g/cm³.

[Preparation of Nonaqueous Electrolytic Solution]

Lithium hexafluorophosphate (LiPF₆) was dissolved with a concentrationof 1.2 mol/L into a mixed solvent including fluoroethylene carbonate(FEC) and methyl 3,3,3-trifluoropropionate (FMP) in a volume ratio of15:85, thereby preparing a nonaqueous electrolytic solution. Further,LiSO₃F was added in an amount of 1 mass % relative to the total amountof the nonaqueous electrolytic solution.

[Fabrication of Exterior Case]

An exterior case was fabricated in the following manner. First, an ironbase plate having a Ni-plated surface was drawn to form a bottomedcylindrical exterior case 5. Next, the exterior case 5 was processed toform a circumferential groove 5 c that was U-shaped in cross section andhad a width of 1.0 mm and a depth of 1.5 mm near the opening of thecylinder. The plate thickness of the cylindrical portion of the exteriorcase 5 was 0.25 mm, and the plate thickness of the bottom 5 a of theexterior case 5 was 0.3 mm. The diameter of the bottom 5 a was 18 mm.SEM observation showed that the thickness of the Ni plating on the innersurface of the bottom 5 a of the exterior case 5 was not more than 2 μm.

[Fabrication of Battery]

The positive electrode 1 and the negative electrode 2 were woundtogether through a microporous polyethylene film as a separator 3 toform an electrode assembly 4. The nonaqueous electrolytic solution andthe electrode assembly 4 were placed into the exterior case 5 whileconnecting the positive electrode 1 in the electrode assembly 4 to afilter 12 through a positive electrode lead 15 and the negativeelectrode 2 to the bottom 5 a of the exterior case 5 through a negativeelectrode lead 16. Thereafter, the open end of the exterior case 5 wastightly closed with a sealing member 6 through a gasket 7. In thismanner, a 18650 cylindrical nonaqueous electrolyte secondary battery A1having a designed capacity of 2300 mAh was fabricated.

[Initial Efficiency]

At an environment temperature of 25° C., the battery was charged at aconstant current of 1150 mA [0.5 lt] until the battery voltage reached4.1 V and was further charged at a constant voltage of 4.1 V until thecurrent value reached 46 mA. After a rest of 10 minutes, the battery wasdischarged at 1150 mA [0.5 lt] to a battery voltage of 3.0 V and wasallowed to rest for 20 minutes. The initial efficiency was obtainedusing the following equation:

Initial efficiency=Discharge capacity/Charge capacity×100

[Discharge Characteristics at −5° C.]

At an environment temperature of 25° C., the battery was charged at aconstant current of 1150 mA [0.5 lt] until the battery voltage reached4.1 V and was further charged at a constant voltage of 4.1 V until thecurrent value reached 46 mA. After a rest of 10 minutes, the battery wasdischarged at 1150 mA [0.5 lt] to a battery voltage of 3.0 V and wasallowed to rest for 20 minutes. Next, the battery was charged again at aconstant current of 1150 mA [0.5 lt] until the battery voltage reached4.1 V and was further charged at a constant voltage of 4.1 V until thecurrent value reached 46 mA. Thereafter, the environment temperature waschanged to −5° C., and the battery was discharged at 1150 mA [0.5 lt] toa battery voltage of 3.0 V. The retention of discharge capacity at −5°C. was determined according to the following equation:

Retention of discharge capacity at −5° C.=Discharge capacity at −5°C./Discharge capacity at 25° C.×100

[High-Temperature Storage Characteristics]

At an environment temperature of 45° C., the battery was subjected to600 cycles of charging and discharging under the same charging anddischarging conditions as in the testing of the initial efficiency. Theretention of capacity after the 600 cycles was calculated using thefollowing equation. Here, the retention of capacity after the 600 cyclesat 45° C. is an index for the evaluation of high-temperature storagecharacteristics.

Retention of capacity=(Discharge capacity in 600th cycle/Dischargecapacity in 1st cycle)×100

[High-Temperature Over-Discharging]

A ceramic resistor was connected to the positive and negative electrodesof the battery to form an external short-circuit. The battery was thenstored in a thermostatic chamber at 60° C., and the appearance of thebattery after 10 days was observed.

Example 2

A battery A2 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 1,except that vinylene carbonate (VC) was further added to the nonaqueouselectrolytic solution in an amount of 1 mass % relative to the totalmass of the nonaqueous electrolytic solution.

Comparative Example 1

A battery C1 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 1,except that LiSO₃F was not added.

Comparative Example 2

A battery C2 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in ComparativeExample 1, except that vinylene carbonate was added to the nonaqueouselectrolytic solution in an amount of 1 mass % relative to the totalmass of the nonaqueous electrolytic solution.

Example 3

A battery A3 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 2,except that the nonaqueous solvent was changed to a mixed solventincluding FEC, PC and FMP in a volume ratio of 15:5:80.

Comparative Example 3

A battery C3 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 3,except that LiSO₃F was not added.

Example 4

A battery A4 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 2,except that the nonaqueous solvent was changed to a mixed solventincluding FEC, PC, FMP and ethyl methyl carbonate (EMC) in a volumeratio of 15:5:60:20.

Comparative Example 4

A battery C4 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 2,except that the nonaqueous solvent was changed to a mixed solventincluding FEC and EMC in a volume ratio of 15:85.

Comparative Example 5

A battery C5 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in ComparativeExample 4, except that LiSO₃F was not added.

Comparative Example 6

A battery C6 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in Example 2,except that the nonaqueous solvent was changed to a mixed solventincluding ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in avolume ratio of 15:85.

Comparative Example 7

A battery C7 was fabricated and was tested to evaluate the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in the same manner as in ComparativeExample 6, except that LiSO₃F was not added.

Table 1 describes the results of the evaluations of the initialefficiency, the discharge characteristics at −5° C., thehigh-temperature storage characteristics and the high-temperatureover-discharging characteristics in Examples 1 to 4 and ComparativeExamples 1 to 7.

TABLE 1 High-temperature Discharge storage Appearance after Initialcharacteristics characteristics 10 days of over- efficiency at −5° C.vs. during cycles at discharging at high Solvent Additives (%) 25° C.(%) 45° C. (%) temperature Ex. 1 FEC/FMP (15/85) LiSO₃F 1 wt % 88 82 88No change Ex. 2 VC 1 wt % + 88 82 89 No change LiSO₃F 1 wt % Comp. Notadded 85 82 89 No change Ex. 1 Comp. VC 1 wt % 85 83 89 No change Ex. 2Ex. 3 FEC/PC/FMP VC 1 wt % + 89 83 88 No change (15/5/80) LiSO₃F 1 wt %Comp. VC 1 wt % 84 83 88 No change Ex. 3 Ex. 4 FEC/PC/FMP/EMC VC 1 wt% + 88 83 88 No change (15/5/60/20) LiSO₃F 1 wt % Comp. FEC/EMC (15/85)VC 1 wt % + 86 81 88 Case corroded Ex. 4 LiSO₃F 1 wt % Comp. FEC/EMC(15/85) VC 1 wt % 87 79 86 No change Ex. 5 Comp. EC/EMC (15/85) VC 1 wt% + 87 79 76 Case corroded Ex. 6 LiSO₃F 1 wt % Comp. EC/EMC (15/85) VC 1wt % 87 72 70 No change Ex. 7

As described in Table 1, the comparison of Examples 1 and 2 toComparative Examples 1 and 2 which involved the same nonaqueous solvent(FEC/FMP) has confirmed that the addition of LiSO₃F improves the initialefficiency as demonstrated in Examples 1 and 2. Further, the comparisonof Example 3 to Comparative Example 3 has shown that the initialefficiency was improved in Example 3 and thus has confirmed that theaddition of LiSO₃F to FEC/PC/FMP as the nonaqueous solvent also improvesthe initial efficiency. From the comparison of Examples 1 and 2 andComparative Examples 1 and 2 to Example 3 and Comparative Example 3, thedegree of the enhancement in initial efficiency was greater betweenExample 3 and Comparative Example 3. This result is probably because PCpresent in the nonaqueous solvent used in Example 3 and ComparativeExample 3 reacted coordinately with FEC and FMP to form a densecomposite film which contributed to the greater enhancement.

Example 4, which involved EMC in addition to the nonaqueous solvent usedin Example 3, resulted in an improvement in initial efficiency similarlyto Example 3.

The nonaqueous solvents used in Examples 1 to 4 and Comparative Examples1 to 3 contained FMP, and consequently the batteries exhibited excellentlow-temperature discharge characteristics and high-temperature storagecharacteristics. In contrast, the absence of FMP in the nonaqueoussolvent in Comparative Examples 4 and 5 caused a decrease inlow-temperature discharge characteristics. Further, the nonaqueoussolvent in Comparative Examples 6 and 7 did not contain FMP and thebatteries were poor in both low-temperature discharge characteristicsand high-temperature storage characteristics.

The observation of the appearance of the batteries showed that theexterior cases had been corroded in Comparative Examples 4 and 6 inwhich the nonaqueous electrolytic solution contained LiSO₃F but was freefrom FMP, whilst the appearance of the other batteries was unchanged.These results have confirmed that the addition of FMP prevents thecorrosion of the exterior case even in the case where the nonaqueouselectrolytic solution contains LiSO₃F.

Here, it is desirable that the exterior case 5 have a thin portion 5 bin the bottom 5 a. However, the formation of such a thin portion 5 b inthe exterior case 5 tends to cause the thickness of the Ni plating to bereduced. Such thinned Ni plating will be consumed by the reaction ofLiSO₃F-derived decomposition products with nickel, and consequently ironwill be exposed and the exterior case will be corroded. In order toconfirm whether the corrosion of the exterior case 5 would be preventedeven in the thin portion 5 b, batteries of Example 5 and ComparativeExample 8 were fabricated and were over-discharged at a hightemperature. In the evaluation of the high-temperature over-dischargingcharacteristics, the appearance was observed after 5 days and after 10days.

[Formation of Thin Portion]

In the fabrication of batteries of Example 5 and Comparative Example 8,a thin portion 5 b having the following dimension was formed in theexterior case 5. As illustrated in FIG. 3, the thin portion 5 b was acircular ring with a diameter D of 9 mm that was recessed from the levelof the outer bottom surface toward the inner surface of the bottom. Theplate thickness of the thin portion 5 b was 0.25 mm. SEM observationshowed that the thickness of the Ni plating on the inner surface of thethin portion 5 b of the exterior case 5 was not more than 1 μm. Theproportion of the area of the circular region enclosed by the thinportion 5 b was 25% relative to the area of the bottom 5 a.

Example 5

A battery A5 was fabricated and was tested to evaluate thehigh-temperature over-discharging characteristics in the same manner asin Example 2, except that a thin portion 5 b was formed in the bottom 5a of the exterior case 5.

Comparative Example 8

A battery C8 was fabricated and was tested to evaluate thehigh-temperature over-discharging characteristics in the same manner asin Comparative Example 4, except that a thin portion 5 b was formed inthe bottom 5 a of the exterior case 5.

Table 2 describes the results of the evaluation after 5 days or 10 daysof over-discharging at high temperature in Example 5 and ComparativeExample 8 together with the results in Example 2 and Comparative Example4.

TABLE 2 Appearance Appearance after 5 after 10 Thin days of over- daysof over- portion discharging discharging in at high at high SolventAdditives bottom temperature temperature Ex. 5 FEC/FMP VC 1 wt PresentNo change No change Ex. 2 (15/85) % + Absent No change No change LiSO₃F1 wt % Comp. FEC/EMC VC 1 wt Present Case Case Ex. 8 (15/85) % +corroded corroded Comp. LiSO₃F Absent No change Case Ex. 4 1 wt %corroded

In Comparative Example 8, as described in Table 2, the thin portion 5 bof the exterior case 5 was found to have been corroded during the 5 daysof over-discharging at high temperature. In Comparative Example 4 inwhich the nonaqueous solvent did not contain FMP and the battery did nothave any thin portion 5 b, the exterior case was found to have beencorroded during the 10 days of over-discharging at high temperature. Incontrast, the exterior cases 5 in Examples 2 and 5 in which thenonaqueous solvent contained FMP were free from corrosion regardless ofthe presence or absence of the thin portion 5 b. These results show thateven in the case where the nonaqueous electrolytic solution containsLiSO₃F, the incorporation of FMP into the nonaqueous solvent preventsthe corrosion of the exterior case 5 through the formation of a film onthe inner surface of the exterior case 5 as a result of the reactionrepresented by the reaction formula (II). This film probably serves as aprotective layer on the inner surface of the exterior case 5 so as tosuppress the reaction between LiSO₃F-derived decomposition products andnickel and thereby to prevent the exterior case 5 from being corroded.

According to the embodiment described hereinabove, the nonaqueouselectrolytic solution including a fluorinated carboxylate ester andlithium fluorosulfate salt (LiSO₃F), and the nonaqueous electrolytesecondary battery including the nonaqueous electrolytic solution achieveexcellent low-temperature discharge characteristics and high-temperaturestorage characteristics and also attain an improvement in initialefficiency.

In a configuration in which the negative electrode 2 is electricallyconnected to the exterior case 5 and the exterior case 5 contains iron,the nonaqueous electrolyte secondary battery which includes thenonaqueous electrolytic solution containing a fluorinated carboxylateester and lithium fluorosulfate salt (LiSO₃F) is prevented from thecorrosion of the exterior case.

The configurations of the nonaqueous electrolytic solutions fornonaqueous electrolyte secondary batteries of the present disclosure arenot limited to those described in the above embodiment and, for example,the following configurations are also within the scope of the presentdisclosure.

[Item 1]

A nonaqueous electrolytic solution for nonaqueous electrolyte secondarybatteries including a nonaqueous solvent and an electrolyte,

the nonaqueous solvent including a fluorinated carboxylate esterrepresented by the formula (1):

where R₁ and R₂ are each any of H, F, CH_(3-x)F_(x) (x is 1, 2 or 3) andR₃ is an optionally fluorinated alkyl group having 1 to 3 carbon atoms,

the nonaqueous electrolytic solution further including lithiumfluorosulfate salt (LiSO₃F).

[Item 2]

The nonaqueous electrolytic solution described in Item 1, furtherincluding fluoroethylene carbonate (FEC).

[Item 3]

The nonaqueous electrolytic solution described in Item 1, furtherincluding propylene carbonate (PC).

[Item 4]

The nonaqueous electrolytic solution described in Item 1 or 2, whichcontains the LiSO₃F in an amount of 0.1 to 3 mass % relative to thetotal mass of the nonaqueous electrolytic solution.

[Item 5]

The nonaqueous electrolytic solution described in any one of Items 1 to3, wherein the fluorinated carboxylate ester is methyl3,3,3-trifluoropropionate (FMP).

[Item 6]

The nonaqueous electrolytic solution described in any one of Items 1 to4, which contains the fluorinated carboxylate ester in an amount of notless than 50 vol % relative to the total volume of the nonaqueoussolvent in the nonaqueous electrolytic solution.

[Item 7]

The nonaqueous electrolytic solution described in any one of Items 1 to6, further including vinylene carbonate (VC) in an amount of 0.1 to 3mass % relative to the total mass of the nonaqueous electrolyticsolution.

[Item 8]

A nonaqueous electrolyte secondary battery including an electrodeassembly in which a positive electrode and a negative electrode arestacked together through a separator, a nonaqueous electrolyticsolution, and an exterior case accommodating the electrode assembly andthe nonaqueous electrolytic solution,

the negative electrode being electrically connected to the exteriorcase,

the exterior case containing iron,

the nonaqueous electrolytic solution including a fluorinated carboxylateester and lithium fluorosulfate salt (LiSO₃F).

[Item 9]

The nonaqueous electrolyte secondary battery described in Item 8,wherein the exterior case has a thin portion in a bottom.

[Item 10]

The nonaqueous electrolyte secondary battery described in Item 8 or 9,wherein the exterior case is plated with nickel and the nickel platinghas a thickness of not more than 1 μm in at least a portion of an innersurface of the exterior case.

[Item 11]

The nonaqueous electrolyte secondary battery described in any one ofItems 8 to 10, wherein the positive electrode includes alithium-containing transition metal oxide as a positive electrode activematerial and the proportion of nickel in the total of the transitionmetal(s) is not less than 50 mol %.

What is claimed is:
 1. A nonaqueous electrolytic solution for nonaqueouselectrolyte secondary batteries comprising a nonaqueous solvent and anelectrolyte, the nonaqueous solvent including a fluorinated carboxylateester represented by the formula (1):

where R₁ and R₂ are each any of H, F, CH_(3-x)F_(x) (x is 1, 2 or 3) andR₃ is an optionally fluorinated alkyl group having 1 to 3 carbon atoms,the nonaqueous electrolytic solution further comprising lithiumfluorosulfate salt (LiSO₃F).
 2. The nonaqueous electrolytic solutionaccording to claim 1, further comprising fluoroethylene carbonate (FEC).3. The nonaqueous electrolytic solution according to claim 1, furthercomprising propylene carbonate (PC).
 4. The nonaqueous electrolyticsolution according to claim 1, which contains the LiSO₃F in an amount of0.1 to 3 mass % relative to the total mass of the nonaqueouselectrolytic solution.
 5. The nonaqueous electrolytic solution accordingto claim 1, wherein the fluorinated carboxylate ester is methyl3,3,3-trifluoropropionate (FMP).
 6. The nonaqueous electrolytic solutionaccording to claim 1, which contains the fluorinated carboxylate esterin an amount of not less than 50 vol % relative to the total volume ofthe nonaqueous solvent in the nonaqueous electrolytic solution.
 7. Thenonaqueous electrolytic solution according to claim 1, furthercomprising vinylene carbonate (VC) in an amount of 0.1 to 3 mass %relative to the total mass of the nonaqueous electrolytic solution.
 8. Anonaqueous electrolyte secondary battery comprising an electrodeassembly in which a positive electrode and a negative electrode arestacked together through a separator, a nonaqueous electrolyticsolution, and an exterior case accommodating the electrode assembly andthe nonaqueous electrolytic solution, the negative electrode beingelectrically connected to the exterior case, the exterior casecontaining iron, the nonaqueous electrolytic solution including afluorinated carboxylate ester and lithium fluorosulfate salt (LiSO₃F).9. The nonaqueous electrolyte secondary battery according to claim 8,wherein the exterior case has a thin portion in a bottom.
 10. Thenonaqueous electrolyte secondary battery according to claim 8, whereinthe exterior case is plated with nickel and the nickel plating has athickness of not more than 1 μm in at least a portion of an innersurface of the exterior case.
 11. The nonaqueous electrolyte secondarybattery according to claim 8, wherein the positive electrode includes alithium-containing transition metal oxide as a positive electrode activematerial and the proportion of nickel in the total of the transitionmetal(s) is not less than 50 mol %.